Genetic parameter estimates for total weight of lamb weaned in Afrino and Merino sheep

LivestockProductionScience48 (1997)

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Genetic parameter estimates for total weight of lamb weaned in Afrino and Merino sheep M.A. Snyman a**,J.J. Olivier ‘, G.J. Erasmus b, J.B. van Wyk b a GrootfonteinAgricultural DecelopmentInstitute, Pricate Bag X529. Middelburg Cape. 5900. SouthAfrica b Departntentof Animal Science. Unicersity of the OFS, Box 339. Bloemfontein.9Mo. SouthAfrica Accepted 14 November 1996

AbSttXCt A study was undenaken to estimate genetic parameters for total weight of lamb weanal in

Atiiuo and Merinosheep

Data collected on the Camarvon Aftino flock, the Camarvon Merino flock and the Gmotfomein Met& stud were used For Afrino sheep,heritability estimatesof 0.061 (0.043). 0.054 (0.055) and 0.170 (0.074) wm &aimed for total weight of lsrnb weaned after the first (TWWI), second (TWW2) and third (TWW3) parities respectively. ComJponding estimstes for CarnsrvonMerino sheep ranged from 0.090 (0.025) for TWWI to 0.257 (0.047) for TWW4. Similar estimates of O.OIt4 (0.027), 0.045 (O&29), 0. I32 (0.048) and 0.100 (0.054) were obtainedfor GrootfonteinMerino sheep.High pusitive genetic

(ro) aad phenotypic(rJ correlationswere estimatedbetween first parityand lifetime mpmducbveperfonnsncein aI1thme flocks. For tbe Camarvon Afrino flock, ro increased from 0.609 (tI.367) between TWWI

and TRW2 to 0.791 (0.206) and TWW3. The corresponding rP, however, decreasedfrom 0.735 (0.019) to 0.613 (0.026k Shnifar &creases in rp from 0.708 (0.01 I) and 0.6% (0.013) (between TWWI and TWW2) to 0.549 (0.017) aad 0.510 (0.022) (between TWWI and TWW4) were observed for the Camarvon and Grootfontein Merirto f&&s mspectivefy. Genetic correlation estimatesof 0.866 (0.127). 0.913 and 0.736 (0.272) were obtained behveen TWWl and TWWZ. TWW3 and TWW4 for the GrootfonteinMerino flock. The resultsof this study, obtainedwith two different breedsand in two difkreM envimntttetttsin flocks with a high and a low reproductiverate, indicate that selectionfor incmaacdlifethne mpm&ctive performance could be based on total weight of lamb weaned after the first parity. 8 1997 Elsevier 8cience B.V.

between IWWl

Keywords: Sheep:Genetic parameters:Ewe productivity;Total weight of lamb weaned

1. Introduction In almost any sheep enterprise, reproductive performance is of utmost importance in the efficiency of sheep production. This is even more so in the case of mutton and dual purpose sheep breeds. In these

’Corresponding author. Tel.: +^‘4924-21113: 4924-24352; e-mail: [email protected].

fax: +27-

breeds, total weight of lamb weaned per year is the best single measure of a flock’s pmductivity. Nutnerous studies involving the componem traits of reproduction, i.e. fertility, litter size, lamb survivaI rate and number of lambs born and weaned per ewe ioined. have been done to obtain heritability esti*mates‘for these individual traits 6ee Fogatty, 1995, for a review of these studies). The compnsite trait, total weight of lamb weaned per ewe joined, has,

0301-6226/97/$17.00 8 1997 Elsevier Science B.V. All rights reserved. PII SO301-6226(96)01418-2

however, received much less attention (Fogarty, 1995). Lambing percentages in excess of 150% under harsh. extensive conditions and in wool producing sheep breeds lead to the production of a high quantity of lambs, but rhe quality of these lambs are in many instances not acceptable. In view of the limited natural resources, an increase in number of lambs is not the answer to generate higher income from a specific farming enterprise. Selection for increased reproductive performance in such flocks should be aimed at increasing the quality and monetary value of the product in terms of weight and carcass quality.

The aim with slaughter lamb production under extensive conditions is to produce slaughter lambs that can be marketed as soon as possible after weaning, without the need for supplementary feeding. Selection for littersize, without taking the weaning weight of the individual lambs into consideration, would be short sighted. Furthermore, litter size is directly related to ovulation rate, which in turn is influenced by only a few hormones (Hafez et al., 1980) and the responsible genes. Total weight of lamb weaned, however, is determined by litter size as well as several other

factors, such as mothering ability, milk production of the ewe and growth potential of the lamb. Accordingly, the genes influencing these traits would have art effect on total weight of lamb weaned. Selection for reproduction should be based on a criterion closely resembliog the true breeding objective, which is the total weight of lamb weaned per ewe joined. If selection is aimed solely at increasing litter size, the frequency of genes affecting total weight of lamb weaned would perhaps not be sufficiently influenced. Snyman et al. (1996) estimated genetic cotrelations of 0.828 (0.091) and 0.837 (0.072) between lifetime total weight of lamb weaned and lifetime number of lambs born and weaned respectively. These estimates indicate that selection for littersize would increase total weight of lamb weaned, which could be expected, as littersize is the major determinant of total weight of lamb weaned. However,

genetic correlation estimates of weaning weight with lifetime total weight of lamb weaned (0.7521, lifetime number of lambs born (-0.011) and lifetime number of lambs weaned (0. I 11). indica!e that selectiort for littersize would not increase the individual

weaning weight of each lamb, which is just as important as the number of lambs weaned (Snyman et al., 1996). Selection for total weight of lamb weaned would, however, result in a correlated genetic increase in weaning weight of the individual lambs as well. Almost all the heritability estimates for weight of lamb produced cited in the literature were obtained with paternal-halfsib or regression methods (Basuthakur et al., 1973; More G’Ferrall, 1976; Martin et al., 1981: Fogarty et al., 1985; Owen et al., 1986; Abdulkhaliq et al., 1989; Long et al., 1989; Boujenane et al., 1991). However, with an analysjs under an animal model, the full relationship matrix is exploited and any genetic variance due to the dam would also bc accounted for in the estimation of heritability. The objective of this study was to obtain heritability estimates for total weight of lamb weaned in Afrino and Merino sheep by applying restricted maximum likelihood procedures (REML) under an animal model. Genetic improvement of lifetime reproductive performance is not practical by direct selection, but is dependent upon selection for correlated traits. Therefore the genetic correlation between total weight of lamb weaned at the first and subsequent parities was also estimated to ascertain whether selection cannot be performed earlier.

2. Material and methods 2.1. Data Data collected on the Carnarvon Aftino flock (from 1972 to 19941, the Carnarvon Merino flock (from 1962 to 1983) and the Grootfontein Merino stud (from 1966 to 1993) were used for this study. Detailed descriptions of the management and selection procedures followed in these flocks are given by Snyman et al. (1995) for the Afrino flock, Erasmus et al. (1990) for the Camarvon Merino flock and Olivier (1989) for the Grootfontein Merino stud. The Camarvon Afrino and Merino flocks were kept on natural pasture at the Departmental Experimental Station near Camarvon (30’59’ S, 22’9’ E) in the North-western Karen region of the Republic of South Africa. The vegetation consists of mixed grass

and karoo shrub and is described as arid karoo. The

average annual rainfall is 209 mm and it occurs mainly during the autumn months. The official grazing capacity is 5.5 hectare per small stuck unit. The Grootfontein Merino stud is kept at Grootfontein Agricultural Development Institute near Middelburg (31”28’S, 25” I’E) in the North-eastern Karoe region. The stud is run under favorable nutritional conditions, which include irrigated pastures and supplementary feeding. Young ewes in all three flocks were mated at I8 months of age for the first time. Average recorded two tooth body weights were 54.13 kg for Afrino sheep (Snyman et al., 199S), 35. I8 kg for Camarvon Merino sheep (Snyman et al., 1996) and 50.39 kg for Grootfontein Merino sheep (Olivier et at., 1994). The average number of lambs born per ewe lambing over the experimental period was I .53 for Carnarvon Afrino, 1.14 for Camarvon Merino and I .56 for Grootfontein Merino sheep. The corresponding number of lambs weaned per ewe joined was 1.27, 0.70 and 1.09respectively. For ah three flocks, the available data for each ewe for each lambing season included identity of the sire of the lamb/s, birth date, identity, birth weight, sex, birth status and 120 day weaning weight of each lamb. From these data, the total weight of lamb weaned for each ewe joined for each lambing season (‘lWW/Ef) was calculated as follows: Firstly, within each lambing season, weaning weight for all lambs was corrected to 120 days, followed by least-squares corrections for sex of the

lamb. No corrections were made for birth status. Secondly, TWW/El was calculated by adding the corrected weaning weight of ah the lambs weaned by each ewe in that specific lambing season. Subsequently, total weight of lamb weaned by each ewe over n lambing opportunities (TWWn; n=l ,*.-, 71, was calculated. For example, total weight of lamb weaned over three lambing opportunides (TWW3) was calculated as the sum of T’WW/EJ for the first, second and third lambing opportunities. For each ewe in the data set, total weights of lamb weaned over one (TWWI), two (TWWZ), three (TWW31, four (TWW41, five (TWWS), six (TWW6) and seven (TWW7) lambing opportunities were calculated, depending upon the

Table I Number of ewe recordsfor each n number of lambing oppotlunilies Trait ’

TWWI TWw2 TWw3 TWw4 TWw5 ‘lWW6 Tww7

No. of ewe records Coon Afrino

CilllWNOll Merino

Gmotfonein Merino

1025 794 6‘la 452 243 0 0

2510 2237 1991 1891 1466 746 93

2570 2073 1616 1195 635 232 27

‘TWWI...., TWW7=tdal weight of lamb wcmxl by ewes consccurivelambing opporruniwhich had at least one.. . . . seven ties respectively.

number of lambing opportunities each ewe bad over her lifetime in the specifK Bock. The numbers of ewe records available for total weight of lamb weaned in the original data sets for the three Backs for each n number of lambing opportunities are summarized in Table I. These in&de only records of ewes which had n consecutive number of lambing opportunities for each respective TWWn. From Table I it is evident that betweea 50 and 75% of the ewes in the three flocks had at least three to four lambing opportunities. For tbe purpose of this study, TWW3 and TWW4 were taken as an indiiation of lifetime reproductive performance for Afrino and Merino ewes respectively. Therefore, the traits TWWI, TWW2 and TWW3 were analyaed for the Camarvon Aftino flock, and TWWI, TWW2, Tww3 and TWW4 were analyzed for the Carnar~~n and Grootfontein Merino flocks. 2.2. Variance compontwt and genetic parameter estimation

(Co)variance components were estimated using the DFREML programme of Meyer (1989. Meyer, 1991, 1993). Full pedigrees were available for all flocks. Single trait animal models, including direct additive genetic effects and a fixed effect for yearseason of birth of the ewe, were fitted for the estimation of heritability forTWW1, TWW2. TWW3 and TWW4. Bivariate animal models were fitted for the estimation of genetic and phemtypic CoidationS among these traits. Approximate sampling errors

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M.A. Snynan er al. / Liwsmck Producriott Science 48 f 1997) 1I I-I 16

were calculated by fitting a quadratic function to the profile likelihood for each parameter (Meyer and Hill, 1992).

3. Results and discussion The number of records, mean. phenotypic standard deviation and coefficient of variation for TWWn, number of lambs born (NLB3) and weaned (NLW3) per ewe joined and weaning weight for individual lambs (WW) for the three flocks are summarized in Table 2. As the mean TWWn included records from ewes that did not lamb or wean a lamb, as well as from ewes that weaned more than one lamb per lambing opportunity, relatively large standard deviations and

No. records

Carnartt~n Afrino 1025 Iwwl 794 Tww2 640 Tww3 640 NLB3 640 NLW3 3978 WW Carnartion Twwl TWW2 TWW3 TWW4 NLB3 NLw3 WW

Merino 2510 2237 1991 1891 1991 1991 8480

Grootfonrein Merino 2570 Twwl 2073 TWW2 1616 TWw3 1195 TWw4 1616 NLB3 1616 NLw3 9657 W-W ‘TWWI....,

Mean (kg)

SD

31.59 73.80 116.80 4.27 3.94 27.67

18.24 25.18 32.53 1.32 I .35 3.63

57.73 34.12 27.85 30.9 I 34.01 13.12

10.03 16.38 21.12 26X 1.00 I .03 3.16

114.42 72.56 55.9 I Jti.Tb 45.05 54.79 15.23

28.20 26.21 33.43 41.27 1.28 I .34 3.99

58.17 41.41 37.06 33.84 3!.92 40.12 15.22

8.77 22.58 37.77 54.3(i’ 2.22 1.88 20.75

48.47 55.34 90.20 121.97 4.01 3.34 26.22

., ’ ;

_

Trait ’

cri

Camarron TWWI TWW2 TWW3

Afrino 20. I25 34.4 I8 179.107

Carnanou TWWI TWW2 TWw3 TWW4

Merino 9.014 44.936 99.927 177.149

Groorfonfein Merino IWWI 66.998 TWWL 31.091 147.178 TWW3 TWW4 170.371

0;

11:

3 12.493 599.494 878.914

332.618 633.913 1058.021

0.061 (0.043) 0.054 (0.055) 0.170 (0.074)

91.607 223.497 345.938 512.301

100.621 268.433 445.865 689.450

728.073 659.223 970.43 I 1532.888

795.070 690.3 I ? I I 17.609 1703.265

Lr;

0.090 0.167 0.224 0.257

(0.025) (0.036) (0.043) (0.047)

0.084 (0.027) 13.045 (0.029) 0.132 10.048) 0.100 (0.054)

’ uA*= direct additive genetic variance. cri = residual variance, u,? = phenotypic variance, hi = direct heritability. b See Table I.

Table 2 Description of the data sets for the three flocks Trait ’

Table 3 Variance components (kg’) and direct heritability estimates for TWWI. ‘lWW2. lWW3 and TWW4 for the three flacks ”

CV8

TWA’4 = to&l weight of lamb we:aed

by ewes

respectively; NLB3 = number of lambs born per ewe joined over three lambing opportunilies: NLW3 = number of lambs weaned per ewe joined over three lambing opportunities: WW = weaning weight of individual lambs.

coefficients of variation were recorded. For all breeds, the coefficients of variation for TWWn decreased from the first to the third and fourth joining. From Table 2 it is evident that TWWn for the Camarvon Merino flock was very low, compared to that of the Camarvon Afrino and Grootfontein Merino flocks. This could be ascribed to the lower number of lambs born and weaned per ewe joined, as well as the lower weaning weight of the individual lambs (Table 2). The main contributing factor was that 57% of the young ewes in' the Camarvon-Merino Bock did not wean a lamb at their first parity, compared to 20% of the Camarvon Afrino and 14% of the Grootfontein Merino young ewes. This was most probably due to their lower body weight at Brst mating (Section 2.1). Estimates of vxxxariance components and heritability for TWWn are presented in Table 3, while the genetic and phenotypic correlations estimated between TWW I and TWW2, TWW3 and TWW4 for the three flocks are given in Table 4. For Afrino sheep, heritability estimates of 0.061, 0.054 and 0.170 were obtained for TWWI, TWW2 and TWW3 respectively. Corresponding estimates for Carnarvon Merino sheep ranged from 0.090 for TWWl to 0.257 for TWW4. Similar estimates of

Table 4 Genetic

TWW3

and phenotypic correluticmr ktween and TWW4 for the three flocks il Tww2

TWWI

TN++3

and

TWWZ,

Tww4

CamclrvonAfrino lwwl r, 0.609 (0.367) rp 0.735 (0.019)

0.791 (0.206) 0.613 (0.026)

Camaroon Merino lwwl r, l.OoOr,, 0.708 (0.01

I)

l.OOO-0.614 (0.014)

1.000 -0.549 (0.017)

GroorfonfeinMerino Twwl r, 0.866 (0.127) rp 0.6% (0.013)

0.913(0.10) 0.566 (0.017)

0.736 (0.272) 0.510 (0.022)

a

r.

= genetic

TWWI,....

correlation,

TWW4

see Table

rP = phenotypic

correlation:

for

I.

0.084, 0.045, 0.132 and 0.100 were obtained for Grootfontein Merino sheep. High positive genetic ( r, ) and phenotypic (rP) correlations were estimated between TWWI and lifetime reproductive performance in all three flocks. The unity genetic correlation estimates obtained for the Carnarvon Merino flock could most probably be ascribed to the fact that the DFREML-programme used for the analysis, forced the estimates within the parameter space. Therefore sampling errors could also not be obtained. The heritability estimates obtained in this study are in accordance with the animal model estimate of 0.13 (0.06) reported by Fogarty et al. ( 1994) for total weight of lamb weaned. The present estimates also fall within the range of other reported paternal-halfsib estimates, namely 0.03 to 0.09 (Basuthakur et al., 19731, 0.25 and 0.30 (More O’Ferrall, !976), 0.14 (0.10) (Martin et al., 1980, 0.06 to 0.09 (Fogarty et al., 1985), 0.13 to 0.28 (Abdulkhaliq et al., 1989), 0.10 (0.16) (Long et al., 19891, 0.08 (0.05) (Boujenane et al., 1991) and 0.14 (0. IO) (Hall et al., 1994). The higher he&abilities estimated for total weight of lamb weaned in the Carnarvon Merino flock, compared to the Camarvon Afrino and Grootfontein Merino flocks, indicated a larger proportional genetic variance in total weight of lamb weaned in this flock. This could possibly be explained through the fact that no selection for reproduction or weaning weight per se had been carried out during the experimental

period in this flock, In the Camarwm Afrino tlack ewes were, however, s&cted on reproductive performance since the start of the experimental period. In later years, ewes in this flock were culled for knu reproductive rate and below average total weight of lamb weaned after two parities. In the Grootfontcin Merinoflock, iO%oftheeweand5O%oftheram lambs were culled at weaning due to Low adjusted 120 day weaning weight from I!%9 to 1985 (olivier, 1989). From 1985 onwards, ewes in this flock that failed to lamb in two consecutive yeas were culled. The highest heritability estimate within each flock for the traits analyzed, was obtained for TWW3 for Carnarvon Afrino and Grootfontein Merino sheep and for TWW4 for the Carnarvon Merino flock. Compared to the generally low heritabi!ity of v ductive traits, the estimates obtained in this study indicate that suffiiient genetic vtiance in lifetime reproductive efficiency is presenl to warrant the use of total weight of lamb weaned as selection &erion. However, it would be impractical to select ewea only after three or four parities. The high genetic corn++ dons estimated between TWWI and TWW3/TWW4 indicate that these traits arc influenced by the same~ genes. Selection on TWWI could therefore be done in order to increase lifetime nproductive performance in the current flock. The composite trait total weight of lamb weaned, which should be the true breeding objective, incorp~ rates the. component traits fertility, prolifxacy, lamb survival and lamb weaning weight. The heritabiiities of these component traits (Fogarty, 1995) are of the same or&r as, or even lower than that estintpeed for lifetime total weight of lamb weaned. Therefore, if the breeding objective is to increase lifetime reproductive efficiency of the flock, the composite trait total weight of lamb weaned should be the seWion criterion.

4. conclusion The results of this study indicate that there is a relatively large phenotypic variation in total weight of lamb weaned, regardless of the reproductive rate of the flock. This variation may have a genetic basis and could

therefore

be exploited

to genetically

in-

crease lifetime reproductive efficiency in any flock.

MA.

II6

Snwan

CI al. /Lirwrock

Production

Ewe selection is aimed at increasing lifetime rcproductive and productive efficiency in the current flock, as well as the genetic merit of future generations. The high genetic and phenotypic correlations estimated between TWWI and future performance indicate that selection based on TWWI will ensure that the highest producerswill be selected and therefore that gains in the cursent flock would be increased. The genetic variance exploited in this way, should also increase the genetic merit of these ewes’ daughters in terms of lifetime reproductive efficiency.

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Hall, D.G., Gilmour. A.R. and Fogurty. N.M.. 1994. Varialion in reproduction and production of Poll Dorset ewes. Aust. J. Agric. Res., 45(2): 415-425. Long, T.E., Thomas, D.L., Fernando, R.L.. Lx&s. J.M., Garrigus, U.S. and Waldron. D.F., l98Y. Estimation of individual and maternal heterosis. repeatability and heritability for ewe productivity and its components in Suffolk and Targhee sheep. J. Anim. Sci., 67: 1208-1217. Martin, T.G., Nicholson, D.. Smith, C. and Sales, D.I., 1981. Phenotypic and genetic parameters for reproductive performance in a synthetic line of sheep. J. Agric. Sci. Camb., 96: 107-113. Me;rer, K.. 1989. Restricted maximum likelihood 10 estimate variance components for animal models with several random effects using a derivative-free algorithm. Genet. Sel. Evol., 21: 317-340. Meyer, K., 1991. Estimating variances and covariances for multivariate animal models by restricted maximum likelihood. Genet. Sel. Evol., 23: 67-83. Meyer, K., 1993. DFREML: Programs to estimate variance components by restricted maximum likelihood using a derivativefree algorithm. User notes, Ver. 2. I. Meyer, K. and Hill. W.G.. 1992. Approximation of sampling variances and confidence intervals for maximum likelihood estimates of variance components. I. Anim. Breed. Gene&, 109: 264-280. More O’Ferrall, G.J.. 1976. Phenotypic and genetic parameters of produclivity in Galway ewes. Anim. Prod., 23: 295-304. Olivier. JJ.. 1989. Genetic and environmental trends in tbc GrJotfontein Merino stud. F’hDthesis. University of the Orange Free State. Blocmfontein. Olivicr, J.J., Erasmus, G.J.. van Wyk, J.B. and Konstaotinov, K.V., 1994. Dir& and maternal variance component estimates for clean fleece weight, body weight and mean fibre diameter in the Gmotfontein Merino Stud. S. Afr. I. Anim. Sci., 24(4X 122-124. Owen, J.B.. Crecs, S.R.E. and Williams. J.C.. 1986. Proliticasy and %-day lamb weight of ewes in the Cambridge sheep breed. Anim. Prod., 42: 355-363. Snyman. MA.. Erasmus. G.E. and van Wyk, J.B., 1995. Nongenetic factors influencing growth and fleece traits in Afrino sheep. S. Afr. 1. Anim. Sci., Z(3): 70-74. Snyman. M.A.. Olivier. JJ. and Olivier, W.J., 1996. Variance components and genetic parameters for body weight and fleece rraits of Merino sheep in an arid environment. S. Afr. I. Anim. Sci., 26(l): 1l-14.